The present invention is directed generally to optical transmission systems. More particularly, the invention relates to adding and/or dropping one or more optical signal wavelengths from a wavelength division multiplexed (WDM) signal in an optical communications system.
The emergence of the Internet as a means for transporting and accessing data combined with continual growth in traditional communications has greatly accelerated the need for high capacity transmission systems. Telecommunications service providers, in particular, have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems.
In optical transmission systems, information is typically transmitted between central processing centers, or points of presence, which are used to collect information being electronically transmitted from a number of smaller distributed locations. In these systems, it is often desirable and cost effective to distribute or collect information along the optical path between the centers without the cost of providing another central processing center. Optical add/drop (“OAD”) devices can be used at locations in the optical system where the amount of information being transmitted and received at the location does not make it economically feasible to process all of the information being transmitted in the system.
Optical add/drop, or insert/remove, devices are generally configured to drop/remove one or more predetermined wavelengths (“drop wavelengths”) from a WDM signal entering the device and add/insert the same, or possibly different, wavelengths to the signal. For example, see U.S. Pat. Nos. 5,283,686, 5,555,118, 5,579,143, 5,600,473, 5,726,785, 5,778,118.
Many OAD devices include one or more filtering elements, i.e., Bragg gratings, Fabry-Perot filters, etc., which are used to either drop signal wavelengths for further processing or merely filter and remove the signal wavelengths from the transmission line. OAD devices that include filtering elements, or filters, allow for the reuse of the filter wavelengths to add wavelengths to the system.
In some OAD devices, the filtering elements can be tuned to vary the wavelength that is being filtered. For example, the properties of a Bragg grating element can be varied to change the central reflective wavelength of the Bragg grating. In lightly populated WDM systems, sufficient bandwidth exists between signal wavelengths that the elements can be tuned to a wavelength not carrying a signal. Thus, a tunable OAD device can be provided in which the dropped or filtered wavelengths can be variably tuned according the requirements of the system.
However, in dense wavelength division multiplexing (“DWDM”) systems, there is not sufficient bandwidth between the signal wavelengths to tune the filtering element in the OAD device to pass a wavelength without interfering with an adjacent wavelength. Therefore, many traditional OAD devices can not operate as tunable devices in DWDM systems.
This limitation of traditional OAD devices affects the flexibility of DWDM systems, particularly when planning for system upgrades or reconfigurations. For example, traditional OAD devices will have to be replaced when new or different wavelengths are to be dropped and/or added depending upon the configuration of the traditional OAD device.
One proposed solution is to place the filtering elements in one leg of an optical line switch and provide a bypass line in another leg of the switch. While bypass line switching provides the desired functionality, the use of line switches can introduce an unacceptable amount of loss into the OAD device. As such, the switched filter OAD devices can not be widely deployed in optical systems. Thus, there remains a clear need for OAD devices that provide increased flexibility and inexpensive upgrade paths for DWDM systems.